1. Field of the Invention
The present invention relates to a method for controlling the composition of aqueous solutions including a chemical copper plating solution, and more particularly to a method for continuously and automatically controlling the pH of a chemical copper plating solution, the concentration of a reducing agent, the concentration of copper ions and the concentration of a complexing agent with good exactness.
2. Brief Description of the Prior Art
In the bright plating of ABS resin for decoration, a copper ground plating film is usually formed on the resin surface by chemical copper plating to give an electroconductivity to the resin surface. In that case, the mechanical strength of the ground plating film is no problem, and thus control of the composition of a chemical copper plating solution having an effect upon a plating film is intermittently carried out.
According to the recent technique, a conductor is directly formed on an insulating substrate by chemical copper plating. In that case, the mechanical characteristics of the plating film are regarded as important in addition to the electroconductivity of the plating film. The mechanical characteristics of a chemical copper plating film depend upon concentration of the main components of a chemical copper plating solution. Thus, it is necessary to continuously control the composition of a chemical copper plating solution.
According to the prior art, continuous control of a chemical copper plating solution is carried out in the following manner (a)-(c) as to pH, concentration of a reducing agent, concentration of copper ions and concentration of complexing agent.
(a) pH: A predetermined volume of a chemical copper plating solution (for example, pH 12.3) is admixed with a predetermined volume of an acid solution of predetermined concentration to adjust the pH to 4-9, and the pH of the solution is measured as a potential by means of glass electrode-calomel electrode. When there is a difference between the measured potential and the potential of fresh plating solution, the difference in potential is transmitted as a signal to actuate a controller, and a makeup solution is added to the plating solution to adjust the pH value of that of the fresh plating solution (Japanese Laid-open Patent Application Specification No. 83635/79).
(b) Concentration of reducing agent: A predetermined volume of a chemical copper plating solution is admixed with sodium sulfite in excess of the necessary amount for reaction with the total amount of formaldehyde as the reducing agent and also with an inhibitor for autodecomposition of sodium sulfite, thereby allowing formaldehyde to react with sulfite ions, and then admixed with iodine and a buffer agent, thereby allowing the unreacted sulfite ions to react with excess iodine, and an equilibrium potential between the remaining iodine and iodine ions is measured by means of gold electrode-calomel electrode. When there is a difference between the measured potential and the potential of the fresh plating solution, a controller is actuated to supply a makeup solution to the plating solution and make the concentration of formaldehyde equal to that of the fresh plating solution (Japanese Laid-open patent application specification No. 1093/79).
(c) Concentrations of Cu.sup.+2 ions and complexing agent: In a chemical copper plating solution, there is a complexing agent in excess of Cu.sup.+2 ions. A predetermined volume of a chemical copper plating solution is admixed with metal ions such as Fe.sup.+3, etc. to form complex compounds among the metal ions, the complexing agent liberated by consumption of Cu.sup.+2 ions due to the plating, and the complexing agent existing in a liberated state from the beginning. A change in potential between the plating solution before the addition of Fe.sup.+3 ions and that thereafter is measured by means of gold electrode-calomel electrode to find the concentration of liberated complexing agent. By subtracting the found concentration value of the liberated complexing agent from the known concentration value of total complexing agents in a fresh chemical copper plating solution, the amount of the complexing agent taking part in complexing with Cu.sup.+2 ions, that is, the amount of Cu.sup.+2 ions, is indirectly determined. When there is a difference between the determined amount and the amount of the fresh chemical copper solution, a controller is actuated to supply a makeup solution to the plating solution and make the amount of Cu.sup.+2 ions equal to that of the fresh plating solution (Japanese Laid-open patent application specification No. 149389/78).
Concentration of the total complexing agents is determined by adding Fe.sup.+3 in an amount large enough to react with all the complexing agents and by measuring the potential by means of gold-electrode-calomel electrode. When there is a difference between the measured amount of total complexing agents and the amount of the fresh plating solution, a controller is actuated to supply a makeup solution to the plating solution and make the amount of the total complexing agents equal to that of the fresh plating solution (Japanese Laid-open patent application specification No. 149389/78).
However, the foregoing procedure (a) has such a disadvantage that the glass of glass electrode is attacked by the chemical copper plating solution, and consequently continuous measurement is only possible for a few hours, and a measurement error will be considerable after about 3 hours.
The foregoing procedures (b) and (c) also have such a disadvantage that, if the pH measured is not exact, [I.sup.- ]/[I.sub.2 ] used in the quantitative determination of formaldehyde fails to serve as an index in the procedure (b), and the reaction of Fe.sup.+3 ions with the complexing agent fails to proceed exactly in the procedure (c), and thus in both procedures (b) and (c) exact control of the concentration of reducing agent, the concentration of Cu.sup.+2 ions and the concentrations of total complexing agents will not be carried out after about 3 hours.